Research Document - 1999/168

Natural mortality rates of adult Pacific herring (Clupea pallasii) from southern British Columbia.

By R.W. Tanasichuk

Abstract

Data for over 904,000 Pacific herring (Clupea pallasi) seined or gillnetted in British Columbia between 1951 and 1998 was used to estimate age- and year-specific adult natural mortality rates. Apparent sampling bias precluded using data for all stocks before 1980 and for northern British Columbian stocks since then. For the southern (West Coast Vancouver Island, Strait of Georgia) stocks, the instantaneous natural mortality rate (M) for adult herring is an increasing exponential function of age. Surplus energy requirements for gonad recrudescence and overwintering appear to cause the death of adult herring. It was found that, based on an experiment using a hypothetical herring population and assessment model or regression estimates of M, the escapement model overestimates survival, relative to the age structured model and the predictive regression, for West Coast Vancouver Island herring; the three mortality estimates generate similar abundance trajectories for Strait of Georgia herring. It is suggested that the described sampling bias be considered by herring stock assessment models.

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